In the manufacture of glassware, particularly of glass bottles, the glassware are formed from gobs of glass. These glass gobs are obtained by forcing molten glass through an orifice in the floor of a refractory trough containing the molten glass from the glass furnace. The glass is forced through the orifice by means of an axially reciprocating plunger above the orifice. Below the orifice the gob is cut off and drops away. The glass gob, while still hot, then undergoes further treatment e.g. the moulding into the desired glassware.
The gobs are cut off by means of shears mounted below the orifice in the refractory trough floor. Two forms of shear construction are used. In most commercial glassware manufacturing plants, the shears comprise opposing shear blades mounted on pivotally connected arms. In use these shear blades move between their open and their closed, cutting positions along arcuate paths in a scissor-type action.
The increased production rates required these days have brought with them high speeds of operation for the apparatus. Moreover gob production is further increased by using multi-gob installations. At high speed applications the scissor-type shears have a tendency to excessive wear in the mechanism. Moreover, with multi-gob installations, more than one pair of opposing shear blades may be mounted on the pivotally connected arms of the shears. With this construction the gobs will not all be cut simultaneously. Thus the gobs will not be delivered to for example the moulds at the same time leading to variations in moulding times.
To overcome these problems there has been proposed the use of straight-line shearing. In straight-line shearing the shear blades reciprocally move along a straight line towards and away from each other between the closed, cutting position and the open position in contrast to the arcuate swinging motion of the previously used shears. With straight-line shearing therefore, by having a plurality of opposing shear blade pairs mounted in parallel, a plurality of gobs may be sheared simultaneously and accordingly delivered to the forming moulds simultaneously.
Straight-line shearing apparatus has for example been proposed in U.S. Pat. Nos. 1,642,966 and 3,996,037 and British Patent Specification No. 1216213. However the reliability of the complex mechanisms disclosed tends to be somewhat low leading to the shearing apparatus being out of operation for unacceptably long periods of time and therefore resulting in unacceptably long periods lost to production. More recently straight-line shearing apparatus has been disclosed in British Patent Specification No: 2014128A. In this apparatus the reciprocatable movement of the shear blades is effected by means of a pneumatic mechanism. Again the mechanism tends to be complex and this of course brings with it unreliability.
British Patent Specification No: 2112378A describes apparatus having a plurality of opposing shear blade pairs mounted on pivotal arms. Thus again this is an example of shears wherein the blades are swung along an arcuate path. According to British Patent Specification No: 2112378A however there is used a somewhat complicated bellcranks mechanism which is said to synchronise the shearing operation so that all gobs are sheared at the same time.
British Patent Specification No: 2123809A describes a shearing mechanism operating in a straight line wherein opposing pairs of shear blades are mounted on reciprocal slides which move towards and away from each other along a linear path. The movement of the slides towards and away from each other is effected by the oscillation of a crank sleeve mounted for oscillation about a vertical axis and having diametrically opposing radially extended arms, each of which is connected to one of the blade bearing slides. When the crank sleeve is oscillated in one direction the blade bearing slides are caused to move towards each other to the closed, cutting position and when the crank sleeve is moved in the reverse direction the blade bearing slides are caused to move away from each other to the open position. The crank sleeve is operated from a shear cam in a manner well known in the art to cause the shear blades to move away from each other to the open position and, upon release of the opening force, a spring moves the crank sleeve and accordingly also the shear blades to the closed, cutting position.
In all these glassware manufacturing operations, the gob shears must of course be synchronised with the gob production, that is with the rate of reciprocation of the plungers. Much attention has been given to achieving this synchronisation. One such control mechanism is described in European Patent Specification No: 0087966.
Generally, whether scissor-type shears or straight-line shearing is used, the higher the rate of gob production the higher the speed of operation of the shears. Thus the shearing forces will be varied with the rate of gob production. Moreover maximum cutting rates are usually of the order of 150 per minute.